Process for imparting a low gloss on aluminum



3,523,843 PROCESS FOR IMPARTING A LOW GLOSS N ALUMINUM Walter G. Zelley, Lower Burrell, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed July 21, 1967, Ser. No. 654,976 Int. Cl. C23f 1/00; C23g 1/14 US. Cl. 156-22 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for reducing the gloss of the surface of an aluminum article. More particularly the invention is directed to a process of reducing the gloss of the surface of an aluminum article by etching the article in an improved etching solution. As generally used herein the term aluminum includes pure aluminum, a commercial grade of aluminum, or an alloy consisting of more than 50 percent aluminum; and the term article means an article in sheet, plate, bar or any other form which can be fabricated from such as-defined aluminum. Usually the aluminum article Will be anodized after etching, or a diffuse reflector type finish might be applied by etching and chemical polishing prior to anodizing.

Heretofore, various methods such as etching, grit blasting, glass bead blasting or other surface roughening treat ments have been used to reduce the gloss of the surface of an aluminum article. However, these known methods have not been altogether satisfactory in producing a uniform gloss on the surface of aluminum articles, such as bright finished aluminum articles used as automobile trim parts, etc.

While it is often desirable that an aluminum article, particularly an ornamental trim article, should have a bright finish, i.e., have a pleasing lustre and a uniform metallic appearance, recent safety features specified for automobile trim parts such as windshield wiper arms, etc., require that such parts shall have a 20 degree gloss value not in excess of 40 units. While at present, this desired low gloss may be obtained on the surface of a buffed or rolled bright finished aluminum article by employing the above described known methods, these methods are costly, and do not consistently produce a uniform bright finish and a concurrent uniform low gloss surface both with and across the roll or buii direction of such bright finished articles.

Accordingly, it is an object of this invention to provide a new and improved process for reducing the gloss of the surface of an article of aluminum.

A further object is to provide a new and improved process for reducing the gloss of the surface of an article of aluminum by etching the article in an etching solution which effectively reduces the gloss value of the surface 3,523,843 Patented Aug. 11, 1970 thereof, but enables the article to retain a uniform bright finished appearance and a uniform gloss.

Other objects, advantages, and features will become more clearly apparent from the following description of the invention.

Generally, the invention concerns a process for reducing the gloss of the surface of an article of aluminum by etching the article in a hot aqueous alkaline solution containing an alkali metal hydroxide, zinc ion, and a sequestering agent which is soluble in alkali metal hydroxide solutions. In the preferred method of practicing the invention, the etching solution is maintained at a temperature of at least 125 F. and consists of 25 to 200 grams per liter of an alkali metal hydroxide, preferably sodium hydroxide in Water, 0.2 to 10 grams per liter of zinc ion, and 1 to grams per liter of an alkali metal hydroxide soluble sequestering agent, preferably in the form of Rochelle salt.

Wherever it appears herein the term alkali metal hydroxide soluble sequestering agent means a sequestering agent useful for sequestering alumina which is soluble in a warm aqueous alkali metal hydroxide solution.

Especially good results are obtained when the etching solution is maintained at a temperature from about F. to F. and consists of 40 to 100 grams per liter of alkali metal hydroxide, 0.5 to 5 grams per liter of zinc ion, and 5 to 50 grams per liter of an alkali metal hydroxide soluble sequestering agent.

The invention is applicable to reducing the gloss value of the surface of either a mill-finished aluminum article; i.e., an aluminum article not subjected to any mechanical or chemical brightening treatment, or a bright finished aluminum article, i.e., an aluminum article subjected to a. brightening treatment, for example, a roll or bufif brightening, prior to etching of the article. The invention is especially applicable to reducing the gloss of the surface of a high reflecting aluminum article such as those prepared from commercial grade aluminum, and aluminum-magnesium alloys containing at least 99.6 percent aluminum.

The etching solution is prepared by simply adding the constituents thereof to a suitable container. For example, 25 to 200 grams per liter of an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, or lithium hydroxide, etc., is placed into the container. The zinc ion constituent is obtained by adding and mixing a sufficieut amount of an alkali metal hydroxide soluble zinc salt; i.e., a zinc salt which is soluble in aqueous alkali metal hydroxide solutions, to the alkali metal hydroxide to provide 0.2 to 10 grams per liter of zinc ion. For example, by adding 1 to 50 grams per liter of a zinc sulfate hydrate (ZnSO -7H O) or an equivalent quantity of zinc oxide, zinc nitrate, zinc acetate, etc., the desired amount of zinc ion is realized. Then 1 to 100 grams per liter of a suitable alkali metal hydroxide soluble sequestering agent which is soluble in aqueous alkali metal hydroxide solutions, for example Rochelle salt, sodium citrate, sodium gluconate, etc., is added and mixed in the alkali metal hydroxide-zinc ion solution. The order of addition of the constituents can be reversed, since the sequestering agent and zinc salts are readily soluble in the alkali metal hydroxide solution. The alkali metal hydroxide soluble sequestering agent controls the precipitation of alumina in the solution which would otherwise result in the formation of a hard, adherent scale on the tank walls and equipment.

The etching solution thus prepared is heated to a temperature of at least 125 F. before the aluminum article 3 is subjected to etching. Preferably, the temperature of the etching solution is maintained between 125 F. to 175 F. However, the etching solution must be maintained at a temperature of at least 125 F. in order to avoid the deposition of a continuous zinc layer onto the surface of the aluminum article, which. deposition would effectively prevent etching and thereby prevent the uniform reduction of the gloss of the surface of the aluminum article.

The aluminum article is then etched in the etching solution. Conveniently, the aluminum article is simply immersed in the prepared hot etching solution for a time sufficient to reduce the gloss of the surface of the aluminum article to the desired degree. The time necessary to obtain the reduced gloss varies with the degree of gloss associated with the surface of the aluminum article prior to etching thereof and the extent to which the gloss is to be reduced. Normally an etching time of from about one minute to about six minutes will be effective in reducing the gloss of the aluminum article.

Alternatively, depending on the shape and form of the aluminum article, the application of the etching solution to the aluminum article may be carried out by any of the other methods known in the art; such as air atomization, wherein the etching solution is impinged upon the object to be etched through an atomizing nozzle. Another commonly employed method is the splash method, wherein a series of paddles beat the surface of the bath and causes the etching solution to be impinged upon the object to be etched as it is held above the bath in a position to receive the splash.

After etching, the aluminum article is generally subjected to a water rinse to remove any residual etching solution from the gloss reduced surface and then dipped in an aqueous nitric acid solution for de-smutting.

The following specific examples are given by way of illustration, and are not to be considered as limiting in any way the scope and spirit of the invention.

EXAMPLE 1 An etching solution was prepared. according to the formulation:

Grams per liter by wt.

Sodium hydroxide 100 Zinc ion (obtained by adding grams per liter by Weight of ZnSO -7H O) 1.1

Rochelle salt 5.0

The solution was then heated and maintained at a temperature of 150 F.

A 3" x 6" x 0.040" plate was cut from a sheet of 5252 aluminum magnesium alloy, which contained about 2.5% Mg, 0.05% Si, 0.05% Fe, 0.10% Cu, 0.10% Mn, 0.05% others and remainder, aluminum. One side of the plate had a mechanically bright finished surface imparted by rolling. The other side of the plate had a mill finish.

The plate was entirely immersed in the heated etching solution for 3 minutes. Then the plate was removed from the etching solution, rinsed in water, and de-smutted in a 50 percent nitric acid solution.

Gloss measurements were made at degrees on both, sides of the sample on a Gardner Gloss Meter in accordance with A.S.T.M. standard testing procedure No. D- 523. The measured values are tabulated below in Table I and compared with the 20 degree gloss values obtained on both sides of the sample prior to etching.

4 EXAMPLE 2 An etching solution was prepared according to the formulation:

Grams per liter by wt.

Sodium hydroxide 50 Zinc ion (obtained by adding 1.25 grams per liter by weight of ZnO) 1.0

Rochelle salt 5.0

TAB LE II 20 gloss measurements Mill finish Bright finish Sample With grain X-grain With grain X-grain Before etching 58 100 100 After etching 12 12 21 20 The finally produced aluminum article may optionally be further treated for the purpose of increasing its wear resistance and durability.

For example, a suitable wear resistance and increased durability may be obtained by anodizing the aluminum article by known anodizing techniques.

The above description is presented in terms of particular and preferred embodiments. It is to be understood that the appended claims are intended to encompass the true scope and spirit of the invention and are not limited to these embodiments.

What is claimed is:

1. A process for reducing the gloss of the surface of an article of aluminum, comprising:

etching said article in an etching solution maintained at a temperature of at least F., said solution consisting essentially of:

25 to 200 grams per liter of an alkali metal hydroxide,

0.2 to 10 grams per liter of zinc ion,

1 to 100 grams per liter of an alkali metal hydroxide soluble sequestering agent, and

water,

thereby producing a surface having a 20 degree gloss value not in excess of 40 units.

2. A process for reducing the gloss of the surface of said article of aluminum according to claim 1 wherein said surface is bright finished prior to the etching of said article and wherein said etching solution contains sodium hydroxide as the alkali metal hydroxide and Rochelle salt as the alkali metal hydroxide soluble sequestering agent.

3. A process for reducing the gloss of the surface of an article of aluminum, comprising:

etching said article in an etching solution maintained at a temperature from about 125 F. to F., said solution consisting essentially of:

40 to 100 grams per liter of alkali metal hydroxide,

0.5 to 5 grams per liter of Zinc ion,

5 to 50 grams per liter of an alkali metal hydroxide soluble sequestering agent, and

water,

thereby producing a surface having a 20 degree gloss value not in excess of 40 units.

4. A process for reducing the gloss of the surface of said article of aluminum according to claim 3 wherein said surface is bright finished prior to the etching of said article and wherein said etching solution contains sodium hydroxide as the alkali metal hydroxide and Rochelle 5 salt as the alkali metal hydroxide soluble sequestering agent.

5. A process for reducing the gloss finish of the surface of said article of aluminum according to claim 3 Wherein said etching solution is maintained at a temperature of 150 F. and wherein said etching solution consists essentially of 50 grams per liter of sodium hydroxide as the alkali metal hydroxide,

1.1 grams per liter of zinc ion,

5 to 10 grams per liter of Rochelle salt, as the alkali metal hydroxide soluble sequestering agent, and

Water.

6 References Cited JACOB H. STEINBERG, Primary Examiner US. Cl. X.R. 

